One of factors which limit an output of an internal combustion engine is knocking. A control for retarding an ignition timing if knocking occurs or if a state is reached where knocking is likely to occur is known as a means for preventing knocking. Further, exhaust gas recirculation (EGR) for reducing a combustion temperature by refluxing a part of exhaust gas to an intake system and setting an atmosphere, where knocking is unlikely to occur, in a combustion chamber is also known.
Exhaust gas discharged from an internal combustion engine includes NOx, which has a property of making knocking likely to occur. Thus, if the exhaust gas discharged from the internal combustion engine is refluxed to an intake system as it is, a desired knocking preventing effect cannot be achieved or rather knocking may become more likely to occur if a NOx concentration is high.
Accordingly, a configuration for providing a post-treatment device for removing NOx in an EGR passage is disclosed in JP2000-282965A.